This invention relates generally to plasma investigation and more particularly to modulation of Langmuir probe plasma diagnostic devices.
The Langmuir probe is an experimental device for the determination of plasma densities and energy distribution functions which has intrinsic diagnostic capabilities that are readily applied to laboratory and ionospheric plasma investigations. In its simplest form, the probe is a metallic electrode of cylindrical, planar, or spherical geometry, which collects current from a plasma when a voltage is applied. The probe's current collection properties, specifically referred to as the probe's current-voltage (I-V) characteristic, yield the basic information on the plasma under investigation. If this current-voltage characteristic is distorted by some perturbing mechanism, the accuracy of the technique can be seriously compromised.
In the conventional approach to Langmuir probe operation, the probe is driven by a continuous voltage sweep such as a linear sawtooth voltage. There is considerable evidence that this standard continuous sweep approach to Langmuir probe diagnostics can lead to serious distortions of the current-voltage characteristic measured in contaminating plasma environments. These distortions can manifest themselves as hysteresis in the current-voltage characteristic. Numerous investigators attribute this behavior to the layering of foreign material on the surface of the probe which results in variations of the effective work function of the probe. If these variations occur during the measurement interval, the current-voltage characteristic is distorted, resulting in erroneous determinations of charged-particle densities and energy distribution functions.
There are two conventional approaches to eliminate or circumvent the problem of surface contamination on Langmuir probes. One approach is to periodically clean the probe surface by ion bombardment or by heating the probe. The second approach allows the existence of a contaminating layer and attempts to circumvent the associated difficulties by sweeping the probe voltage at rates which significantly exceed the time within which the probe work function can change. This approach is to reduce the period of the sweep voltage to a value shorter than the time constant associated with the surface contamination.
The periodic probe cleaning procedure is of limited use because new contamination layers can develop immediately after the cleaning process is ended. In the presence of high sorption rates another cleanup may be necessary within seconds of the preceding cleanup termination. The use of a short period for the sweep voltage finds its basic limitations in values of the effective time constant of the surface contamination layer, which can impose unworkably high sweep rates on the probe voltage. High sweep rates can often be handled in laboratory experiments but difficulties arise in rocket or satellite applications where data rate constraints are imposed by telemetry. At high sweep speeds and low telemetry rates, resolution of the current-voltage characteristic is lowered and the accuracy of measurement is reduced.